A ball screw converts rotational movement between a threaded shaft and a threaded nut into a translational displacement between the threaded shaft and threaded nut.
For some time, ball screws have also been used for vehicle brakes that can be actuated electromechanically. In DE 10 2008 062 180 A1, a combination vehicle brake with electromechanically actuated parking brake is described. The vehicle brake comprises, on one hand, a driving brake that can be actuated hydraulically and, on the other hand, a parking brake device that can be actuated electromechanically. The parking brake device comprises a brake housing in which a hydraulic operating pressure chamber is defined by a brake piston that can be charged with a hydraulic pressurized medium for performing braking procedures, so that the brake piston can be moved in the longitudinal direction of the brake piston for achieving a braking effect. The parking brake device acts by means of gearing on the brake piston, wherein the gearing converts the rotational movement of an electromechanical actuator into a translational movement and causes an actuation of the brake piston for performing a parking brake procedure and holds the brake piston in the actuated position. A part of the gearing is constructed as a ball screw and comprises a threaded shaft and a threaded nut that are in contact with each other via rolling bodies formed as balls.
The ball screw comprises the threaded nut and the threaded shaft that define, with their ball grooves, a helical ball channel that is wound about the longitudinal axis of the threaded shaft and in which balls are arranged in a row of balls. When the ball screw is actuated under loading, the balls roll in the ball channel. When the ball screw is actuated without loading, the balls slide. So that it is always possible for the balls to roll under loading or to slide in the event of non-loading actuation of the ball screw, compressible helical springs are provided that guarantee a rolling path for the balls to roll.
The helical springs are arranged in the threaded nut and two springs are supported with their outer end on the threaded nut. The compressed helical springs have the effect that the balls are displaced into a home position. For an unloaded ball screw, the balls can be displaced along the ball channel, because they merely slide and do not roll on the ball grooves of the threaded shaft and the threaded nut.
In the conventional ball screw, the threaded nut has, on its inner side, a recess for the helical spring, so that the helical spring is supported on a contact surface of the threaded nut. The recess must have a certain size so that the helical spring can be positioned straight and then come into contact with the contact surface. Due to the size of the recess, the helical spring is not enclosed by the internal thread of the threaded nut, so that under unfavorable operating conditions there is the risk of buckling. One example for such an operating state is a load in which the balls roll and there is simultaneously a large stroke (travel).